A thermosetting powder coating composition has been used widely in the field of coating for industrial use because it is free from an environmental problem since an organic solvent is not used and because it gives a coating film having excellent characteristics. Powder, coating compositions such as an acrylic resin and polyester are known as such a thermosetting powder coating composition.
However for example, an acrylic resin powder coating composition has merits such as a relatively low cost and excellent appearance of a coating film, but weather resistance thereof is insufficient. Also there is a problem that in case where the same coating line is used also for other resin powder coating composition, an appearance of a coating film is lowered by mixing of a small amount of the other resin powder coating composition. Therefore the coating line must be isolated for each of the coating compositions.
A polyester powder coating composition has also merits such as a relatively low cost, good pigment dispersibility and excellent appearance of a coating film, but there are problems that weather resistance thereof is more insufficient than the acrylic resin powder coating composition and is difficult to be used for coating for outdoor use.
As a powder coating composition being excellent in weather resistance, there is known a thermosetting fluorine-containing resin powder coating composition (JP-B-6-104792, JP-A-6-345822, JP-A-6-184243, etc.). However the thermosetting fluorine-containing resin powder coating composition is high in cost, insufficient in adhesion to a substrate and inferior in an appearance of a coating film, particularly smoothness thereof and also has a problem with storage stability because blocking easily occurs.
In order to eliminate those problems, studies have been made. For example, in case of a polyester powder coating composition, there are attempts that neopentyl glycol is used as a polyol component and isophthalic acid is used as a polycarboxylic acid component in order to enhance weather resistance (JP-A-9-71738, JP-A-11-100548). However when amounts of neopentyl glycol and isophthalic acid components are increased, weather resistance is enhanced though it is inferior as compared with the fluorine-containing resin, but there is a problem that a flexibility and also physical properties of a coating film are lowered.
There is a proposal to solve those problems by blending a fluorine-containing resin with a polyester resin (Japanese Patent No. 2886914). In that invention, a blend of a fluorine-containing resin having hydroxyl group and polyester resin having hydroxyl group is disclosed. However in such a resin blend, adhesion and storage stability are improved, but a compatibility between a fluorine-containing polymer and a polyester polymer is insufficient, an appearance of a coating film is not improved and also weather resistance is not sufficient.